(1) Field of the Invention
The present invention generally relates to a data decision circuit used in an optical parallel receiving module, an optical parallel receiving module, an optical parallel transmission system and a terminal structure of an optical transmission fiber. In particular, the present invention relates to an optical parallel receiving module, a data decision circuit and a terminal structure of an optical transmission fiber which are used in an optical parallel transmission system substitutable for a system using a coaxial cable.
An optical parallel transmission system is superior to a system using a coaxial cable through which electric signals pass in several characteristics: a transmission speed; a transmission distance; a cross talk between transmission path channels; and the like. Therefore, the optical parallel transmission system is expected as an ATM (Asynchronous Transport Mode) equipment, wiring between/in units in a communication equipment, a high-speed interface between computers or the like.
In the optical parallel transmission system expected to be substitutive for a system using the coaxial cable, it is desired that an allowable skew range between channels be expanded.
(2) Description of the related Art
Since the optical parallel transmission system is miniaturized, the structure thereof is simpler than that of an optical transmission system in a trunk line. The optical parallel transmission system in which the structure is simplified is disclosed, for example, in Japanese Laid Open Paten Application No.5-7182.
FIG. 1 shows an example of a conventional optical parallel transmission system 30 which is simplified as a substitution for a system using the coaxial cable. Referring to FIG. 1, the optical parallel transmission system 30 has a transmission link 12, an array fiber 16 and a receiving link 31. The transmission link 12 converts n-channel data supplied as electric signals and a clock signal CLK into data of optical signals and outputs them. The optical signals output from the transmission link 12 is transmitted through the array fiber 16. The receiving link 31 receives the optical signals from the array fiber 16, converts the optical signals into electric signals and generates n-channel output data and the clock signal.
The transmission link 12 has laser diode driver circuits 13.sub.1 -13.sub.n and laser diodes 14.sub.1 -14.sub.n which correspond to the n-channel data DATA 1-DATA n. The transmission link 12 further has, for a clock signal CLK, a laser diode driver circuit 13.sub.n+1 and a laser diode 14.sub.n+1. The receiving link 31 has photo-diodes 22.sub.1 -22.sub.n+1, pre-amplifiers 23.sub.1 -23.sub.n+1 and limiter amplifiers 24.sub.1 -24.sub.n+1 which corresponds to the received n-channel data and the clock signal. The receiver link 31 further has data decision circuits 32.sub.1 -32.sub.n corresponding to the n-channel data and a reference circuit 25 supplying a threshold voltage to the data decision circuits. Optical connectors are used to connect the laser diodes 14.sub.1 -14.sub.n+1 to the array fiber 16 and to connect the array fiber 16 to the photo-diodes 22.sub.1 -22.sub.n+1.
For the sake of miniaturizing, various functions provided in an optical transmission system in a main line are omitted from the optical parallel transmission system. An APC function for stabilizing an optical output is omitted from the transmission link 12. Functions, such as an AGC function and re-timing function, are omitted from the receiving link 31. Since the re-timing function is omitted from the receiving link 31, the clock sinal CLK is transmitted through a line other than lines for data. In addition, a fixed threshold method is used for the system so that burst signals can be processed and the structure can be simplified.
In order to miniaturize the system, elements are integrated. For example, the laser diode driver circuits 13.sub.1 -13.sub.n+1 are is integrated, and the laser diodes 14.sub.1 -14.sub.n+1 are formed in a laser diode array. In addition, in the receiving link 31, the photo-diodes 22.sub.1 -22.sub.n+1 are formed in a photo-diode array, and the pre-amplifiers 23.sub.1 -23.sub.n+1 and the limiter amplifiers 24.sub.1 -24.sub.n+1 are monolithicaly. Also the data decision circuits 32.sub.1 -32.sub.n are monolithicaly.
Optical fibers integrated in an array fiber 16 is used to transmit signals. In addition, an array-shaped optical connector is used.
A description will now be given of an operation of the optical parallel transmission system.
The laser diode driver circuits 13.sub.1 -13.sub.n+1 respectively supply to the laser diodes 14.sub.1 -14.sub.n+1 driving signals corresponding to the transmission data DATA1-DATAn and the clock signal CLK. The respective laser diodes 14.sub.1 -14.sub.n+1 generate optical signals corresponding to the driving signals supplied from the laser diode driver circuits 13.sub.1 -13.sub.n+1. Thus, the data DATA 1-DATA n and the clock signal CLK all of which are electric signals are converted into optical signals in n+1 channels. The optical signals in the n+1 channels are supplied as transmission signals to the array fiber 16 and are transmitted to the receiving link 31.
The photo-diodes 22.sub.1 -22.sub.n+1 respectively convert the received optical signals in the n+1 channels into electric signals. The pre-amplifiers 23.sub.1 -23.sub.n+1 respectively amplify the received signals supplied from the photo-diodes 22.sub.1 -22.sub.n+1 by a predetermined amplification degree and output them.
The limiter amplifiers 24.sub.1 -24.sub.n+1 each of which is formed of differential amplifiers are connected in multi-stage receive the signals supplied from the pre-amplifiers 23.sub.1 -23.sub.n+1 and a threshold voltage V.sub.ref supplied from the reference circuit 25 as input signals. When the received optical power is large, the limiter amplifiers 24.sub.1 -24.sub.n+1 limit the level of the signals based on the threshold voltage V.sub.ref. As a result, in spite of the received optical power, that is, the levels of the outputs of the pre-amplifiers 23.sub.1 -23.sub.n+1, the levels of input signals of the data decision circuits 32.sub.1 -32.sub.n are constant. Thus, the limiter amplifiers 24.sub.1 -24.sub.n+1 output binary data signals having a constant amplitude, the binary data signals corresponding to the transmission data DATA 1-DATA n.
The data decision circuits 32.sub.1 -32.sub.n latch the data signals supplied from the limiter amplifiers 24.sub.1 -24.sub.n at falling edges of the received clock signal CLK. The latched data signals are output as output data DATAO 1-DATAO n from the data decision circuits 32.sub.1 -32.sub.n. The rising edges and falling edges of the output data DATAO 1-DATAO n is coincident with falling edges of the received clock signal CLK.
FIG. 2A shows a waveform of the output of the pre-amplifiers 23.sub.1 -23.sub.n. Due to the variation of optical outputs of the laser diodes 22.sub.1 -22.sub.n, the variation of losses of the optical connector and the variation of the transmission losses of the array fiber 16, the levels of the optical received signals differ from each other in the respective channels in the receiving link 31. The variation of the received optical signal in the respective channels makes the levels of the output signals of the pre-amplifiers 23.sub.1 -23.sub.n to be different.
FIG. 2A illustrates waveforms of the output signals of the pre-amplifiers 23.sub.1 -23.sub.n in a minimum level receiving case and in a maximum level receiving case. In the minimum level receiving case, the level of the received optical signal is minimum. In the maximum level receiving, the level of the received optical signal is maximum.
In a case where the threshold voltage V.sub.ref which is a fixed value is used as a reference level, the pulse width of the output signal of each of the pre-amplifiers 23.sub.1 -23.sub.n in the minimum level receiving case differs from the pulse width of the output signal in the maximum level receiving case. That is, pulse width distortion OCCURS.
FIG. 2B shows waveforms of output signals of the limiter amplifiers 24.sub.1 -24.sub.n in the maximum level receiving case (a) and in the minimum level receiving case (b). The threshold voltage V.sub.ref is generally set at a level close to a center of the output level of the pre-amplifiers 23.sub.1 -23.sub.n in the minimum level receiving case. Thus, in the minimum level receiving case, the eye opening width is expanded in the waveform of the output signal of the limiter amplifiers 24.sub.1 -24.sub.n, as shown in FIG. 2B(b). In this case, further, an rising edge and a falling edge are crossed, so that the eye opening is in the optimum state.
On the other hand, in the maximum level receiving case, the eye opening width is deteriorated and narrows in the waveform of the output signal of the limiter amplifiers 24.sub.1 -24.sub.n. In this case, further an rising edge and a falling edge are not crossed.
FIG. 3 illustrates an eye opening deterioration caused by the variation of turn-on delay time of the laser diodes 14.sub.1 -14.sub.n. In the optical parallel transmission system 30, in order to lower the dissipation power and to simplify the circuits, the APC function is omitted and the laser diodes 14.sub.1 -14.sub.n are used under a condition in which they are biased by "0" volt. Under this condition, due to the variation of a temperature of each of the laser diodes 14.sub.1 -14.sub.n, the oscillation threshold of each of the laser diodes 14.sub.1 -14.sub.n is varied. Thus, under this condition, even if the laser diodes 14.sub.1 -14.sub.n are driven by using a single pulse current having a waveform as shown in FIG. 3(A), the variation of the turn-on delay time occurs in the output light of the laser diodes 14.sub.1 -14.sub.n due to the variation of the oscillation threshold, as shown in FIG. 3(B). Due to this variation of the turn-on delay time, the falling edges of the optical receiving signal in the receiving link 31 vary. As a result, the eye opening of the output signal of the limiter amplifiers 24.sub.1 -24.sub.n is deteriorated as shown in FIG. 3(C).
In addition, also jitters of the data and clock CLK deteriorate the eye opening.
Furthermore, circuits of the transmission link 12 and the receiving link 31, the array fiber 16 and the like provide a skew (a delay time difference) between signals of channels.
In the data decision circuits 32.sub.1 -32.sub.n, data in the respective channels are latched in each time slot having a period T at falling edges of the single clock CLK reproduced in the receiving link 31. The levels of the latched data are then recognized. Under the condition in which the eye opening is deteriorated and the skew occurs between signals in channels, the data decision circuits 32.sub.1 -32.sub.n have to correctly recognize data.
FIG. 4 illustrates a specified tolerance of the skew between signals in channels. In FIG. 4, meshed portion correspond to ranges in which the eye opening deterioration occurs. It is assumed that the set-up hold time in each of the data decision circuits 32.sub.1 -32.sub.n is equal to "0" so that the set-up hold time in each of the data decision circuits 32.sub.1 -32.sub.n is included in the eye opening deterioration.
FIG. 4(B) shows data having a reference phase which is equal to the phase of the clock CLK. In this case, the center of the period T of data corresponds to the falling edge of the clock CLK.
FIG. 4(A) shows data having the most leading phase in the specified tolerance of the skew. The falling edge of the clock CLK must be not in the ranges in which the eye opening deterioration occurs. Due to the eye opening deterioration, an allowable range of the leading phase is decreased. FIG. 4(C) shows data having the most lagging phase in the specified tolerance of the skew. Due to the eye opening deterioration, the allowable range of the lagging phase is deceased.
As has been described above, in the conventional optical parallel transmission system 30, the specified tolerance of the skew is decreased so as to be less than the eye opening width. Thus, the skew allowed for the transmission link 12, the receiving link 31, and the array fiber 16 becomes small, so that the degree of freedom of the system is decreased.